Nylon composition for use in rotational molding

ABSTRACT

A rotational moldable polyamide composition, a method of preparing the composition, and a method of rotational molding. The composition contains from about 0.001 percent to about 0.5 percent by weight of a copper compound, preferably cuprous iodide. Preferably, the composition contains from about 0.001 percent to about 0.3 percent by weight of potassium iodide.

This application is a continuation, of Ser. No. 684,327, filed on Dec.20, 1984, now abandoned, which is a continuation of application Ser. No.464,836, filed on Feb. 8, 1983, now U.S. Pat. No. 4,508,675, which is adivisional of Ser. No. 256,887 filed on Apr. 24, 1981, now abandoned.

BACKGROUND OF THE INVENTION

This invention is the field of polyamide compositions; moreparticularly, the invention relates to a polyamide composition for usein rotational molding and a method for preparing the rotational moldablecomposition.

Rotational molding, also known as rotomolding, is used in themanufacture of hollow objects from thermoplastics. In the basic processof rotational molding, solid or liquid polymers are placed in a mold.The mold is first heated and then cooled while being rotated about twoperpendicular axes simultaneously. Many polymers can be used in therotational molding process. Popular polymers for this use arepolyolefins such as polyethylene. It is also known to usepolycarbonates, crosslinkable polyethylene, nylon, and other materials.In selecting the rotational molding grade formulation, care must betaken to assure that there will not be thermal degradation during theheating cycle. A general discussion on rotational molding is given inMODERN PLASTICS ENCYCLOPEDIA 1979-1980, Volume 56, No. 10A, beginning atPage 381.

Nylon formulations containing metal halide materials such as cuprousiodide and cuprous iodide plus potassium iodide are well known in theart as evidenced by Great Britain Pat. No. 908,647. Since that time,cuprous iodide had been widely used for thermostabilization of polyamidecompounds.

SUMMARY OF THE INVENTION

This invention relates to a heat stabilized polyamide composition whichhas been found particularly useful for rotational molding, a method ofpreparing the composition, and the method of rotational molding usingthis composition. The heat stabilized polyamide composition comprises apolyamide, and about 0.001 to about 0.5 percent by weight of thepolyamide of a copper compound, preferably cuprous iodide. Preferably,there is from about 0.001 percent to about 0.3 percent by weight of thepolyamide of potassium iodide.

A preferred composition comprises polyepsiloncaprolactam having morechain ends terminated by a carboxylic acid group than by an amine group,and between about 0.01 and about 0.05 percent by weight of the polyamideof cuprous iodide, and from about 0.01 percent to about 0.10 percent,based on the weight of the polyamide, of potassium iodide.

The present invention includes a method for preparing the rotationalmoldable composition. The polyamide and from about 0.001 percent toabout 0.5 percent by weight of the polyamide of a copper compound,preferably cuprous iodide, are physically mixed. This mixture is meltblended, preferably extruded, at a temperature above the polymer meltingpoint and less than about 282° C. (540° F.). The melt blended mixture,preferably in pellet form, can be used to rotationally mold an article.The composition preferably contains from about 0.001 percent to about0.3 percent potassium iodide which can be added to the polyamide priorto the addition of the copper compound or physically mixed and meltblended in the same manner as the copper compound.

The method of rotational molding of the present invention comprisesrotationally molding a polyamide article made from a compositioncomprising a polyamide, preferably having more chain ends terminated bya carboxylic acid group than by an amine group, and from about 0.001percent to about 0.5 percent by weight of the total polyamide of acopper compound, preferably cuprous iodide. Preferably, there is fromabout 0.001 percent to about 0.3 percent, based on the weight of thepolyamide, of potassium iodide. The polyamide is fed into the mold andthe composition heated. The mold is rotated followed by cooling afterwhich the article is removed.

Objects, features and advantages of the present invention will becomeapparent by reference to the following specification.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is a heat stabilized polyamide compositionparticularly useful in rotational molding, a method of preparing thecomposition, and the method of rotational molding using thiscomposition. The composition comprises a polyamide, preferably havingmore chain ends terminated by a carboxylic acid group than by an aminegroup, and from about 0.001 percent to about 0.5 percent by weight ofthe polyamide of a copper compound, preferably cuprous iodide. In apreferred composition, the amount of cuprous iodide is between about0.010 percent and about 0.050 percent by weight of the polyamide, and inthe most preferred composition, there is between about 0.015 percent andabout 0.025 percent, by weight of the polyamide, of cuprous iodide.Preferably, there is from about 0.001 percent to about 0.3 percent andmore preferably, from about 0.01 percent to about 0.1 percent, based onthe weight of the polyamide, of potassium iodide.

The term "polyamide" used in the practice of the present invention isintended to include long chain synthetic polymers which have regularlyrecurring amide groups as an integral part of the main polymer chain;and hence, include amide-ester copolymers. Suitable polyamides can beprepared by polymerization of a difunctional monomer or equivalently,its cyclized lactam (e.g., epsilon-aminocaproic acid or caprolactam,respectively) or by the reaction of a conjugate pair of monomers, forexample, a diamide and a dicarboxylic acid (e.g., hexamethylenediamineand adipic acid), or a linear aminoaliphatic acid such as ω-aminoundecanoic acid.

Suitable polylactams can be produced by the polymerization of lactammonomers of the formula ##STR1## where R is an alkylene group havingfrom 3 to 12 or more carbon atoms, preferably from 5 to 12 carbon atoms.A preferred monomer is epsiloncaprolactam having 5 carbon atoms in thealkylene group. Lactam monomers in addition to epsiloncaprolactaminclude pyrollidone, piperodone, valerolactam, caprylactam, lauryllactam, etc. Also included are copolymers of two or more of the above orsimilar lactam monomers. Suitable diamines useful in the polymerizationof polyamides include propanediamine, hexamethylenediamine,octamethylenediamine, etc. Suitable polycarboxylic acids include acidssuch as adipic acid, pimelic, suberic, sebacic, dodecaneoic, etc. Alsoincluded are copolymers or blends of polyamides of the above categories.

Typically, the number average molecular weight of these polymers isbetween about 10,000 and about 50,000, preferably 15,000 to 40,000 andmore preferably 20,000 to 30,000. This is because mechanical propertiesimprove rapidly until about 20,000, and processing starts to become moredifficult above 30,000.

Polyepsiloncaprolactam is the preferred polyamide for use in the presentinvention. Polyepsiloncaprolactam suitable for use herein can contain avariety of terminal functionality. Preferred terminal functionality isthat containing:

(a) A carboxyl group attached to both ends of the polyamide chain;

(b) A carboxyl group attached to one end and an acetamide group attachedto the other end of the polyamide chain;

(c) An amine group attached to both ends of the polyamide chain; and

(d) A carboxyl group attached to one end and an amino group attached tothe other end of the polyamide chain.

It is preferred that the polyepsiloncaprolactam have more chain endsterminated by acid groups than by amine groups. During thepolymerization to form polyepsiloncaprolactam, a controlled amount ofazelaic acid results in a predetermined excess of polycaprolactam chainends terminated by acid groups. Preferably, there are from 50 percent to90 percent, and more perferably from 60 percent to 80 percent, of theend groups acid terminated. Preferably, the end groups are terminatedwith a dicarboxylic acid, such as azelaic acid. When this is the case,the composition can be rotationally molded in the presence of oxygen.However, it is preferred to use an inert, oxygen-free atmosphere, suchas nitrogen or carbon dioxide, to prevent oxidation even where there isexcess end groups terminated with carboxyl groups. If there are not morepolyepsiloncaprolactam chain ends terminated with acid groups than withamine groups, an inert atmosphere is required for a satisfactory productwith the polyepsiloncaprolactam stabilized with cuprous iodide.Otherwise, the composition is susceptible to oxidation resulting in lossof color and physical properties.

Polyamides other than polyepsiloncaprolactam can be used, butpolyepsiloncaprolactam is preferred. For example, polyhexamethyleneadipamide has a higher melting temperature and flows with moredifficulty than polyepsiloncaprolactam. The polyepsiloncaprolactamprovides a lower melting, easier processing product wherein the pelletsof material flow or knit more readily into each other.

The use of copper compounds with polyepsilon caprolactam results in arotational molded article having improved impact properties. It isbelieved that the interaction of the copper cation with the polymerreduces crystallinity in the final product resulting in improved impactresistance properties.

The copper compound can be an organic or inorganic copper salt with thecopper in cupric or cuprous form. A preferred organic salt is cupricacetate. Preferred inorganic salts are copper halides, including cuprousiodide, and cupric and cuprous chloride. The most preferred copper saltis cuprous iodide. The copper compound reduces crystallinity resultingin high impact properties. Additionally, the above listed preferredcopper compounds provide heat stabilization. There is from about 0.001percent to about 0.5 percent by weight, or preferably from about 0.01 toabout 0.05 percent by weight of the copper compound, preferably cuprousiodide.

The polyepsiloncaprolactam composition of this invention does notrequire the use of a plasticizer for successful rotational molding,although caprolactam can be used as a plasticizer. A small amount ofplasticizer, such as caprolactam monomer, can deposit on the mold andmust be washed off.

When polyepsiloncaprolactam is polymerized, about 8 percent to about 12percent of the caprolactam monomer remains. This monomer can be left inthe "unwashed" polyepsiloncaprolactam to act as a plasticizer, or it canbe washed away with water. The "washed" polycaprolactam contains onlyabout 1 percent to 2 percent of the caprolactam monomer.

Potassium iodide which is water soluble, and water soluble coppercompounds, such as copper acetate can be dissolved in water andimpregnated into the polyepsiloncaprolactam. Alternately, potassiumiodide and water soluble or insoluble copper compounds can be meltblended into the polyepsiloncaprolactam.

The copper compound is preferably melt blended with thepolyepsiloncaprolactam at a temperature lower than about 282° C. (540°F.) and above the melting point of the polymer, preferably from 232° C.(450° F.) to 274° C. (525° F.). This is important to prevent anyreaction which may occur between the copper compound and thepolyepsiloncaprolactam from occurring during the melt blending. Thepolyepsiloncaprolactam and the copper compound, preferably cuprousiodide, are preferably melt blended in an extruder.

During the molding process, the composition is heated to from about 288°C. (550° F.) to about 399° C. (750° F.), and preferably from 302° C.(575° F.) to 352° C. (675° F.). If the temperature is too high optimummolding properties are not attained.

When cuprous compounds are used with unwashed polyepsiloncaprolactam,containing about 8 percent to about 12 percent caprolactam monomer, darkspots appear on the rotationally molded articles. The addition ofpotassium iodide eliminates the appearance of these dark spots. Thepotassium iodide also provides additional heat stabilization. It is,therefore, preferred to use potassium iodide in the composition andprocess of the present invention. Preferably, there is from about 0.001percent to about 0.3 percent, and preferably about 0.01 percent to about0.1 percent by weight of potassium iodide.

The rotational molding process with the above described compositioncomprises the steps of preparing a composition of the polyamide, thecopper compound which is preferably cuprous iodide, and preferablypotassium iodide. The composition is preferably in pellet form. Thiscomposition is fed into the rotational mold. The composition is heatedwithin the mold as it is rotated. Generally, the rotational mold rotatessimultaneously along two perpendicular axes. The mold is heated untilthe pellets within the mold melt and flow together on the inside surfaceof the mold. The mold is then cooled and the molded article is removed.

The polyamide composition for rotational molding can be fed into themold in powder or pellet form. If the polyepsiloncaprolactam particlesare too small, their surface area is so large that exposure to theatmosphere results in excessive moisture pickup. For example, a 35 meshsize polyepsiloncaprolactam powder exposed to the atmosphere isunsatisfactory unless it is kept dry. It is preferred to use pelletswhich are not as sensitive to moisture pickup. If thepolyepsiloncaprolactam particles are too large, it takes a longer timefor them to melt and thinner parts cannot be made. Pellets can be fromabout 1/32 by 1/32 inch (or about 1/32 inch average diameter) to about1/8 by 1/8 inch (or about 1/8 inch average diameter). A preferred pelletshape is a cylindrical pellet from about 1/32 to 1/16 inch in diameter,by about 1/16 to 1/8 inch long.

The composition of the present invention can be processed in mostcommercial rotational molding machines. The temperatures range duringthe heating step from about 288° C. (550° F.) to about 399° C. (750°F.), and preferably from about 302° C. (575° F.) to about 352° C. (675°F.). If the temperature is too high during rotational molding, theimpact properties deteriorate. The temperature must be high enough forthe pellets to fuse together to form a smooth inner surface of themolded article. The mold is heated by suitable means known in the art.Generally, the mold rotates within a forced air circulating oven.

After the heating step the mold is cooled. The part must be cool enoughto be easily removed from the mold and retain its shape. Preferably, themold is removed from the oven while continuing to rotate. Cool air isfirst blown on the mold. The air can be at ambient temperature. Afterthe air has started to cool the mold for a controlled time period, awater spray can be used. The water cools the mold more rapidly. Thewater used can be at cold water tap temperature, usually from about 4°C. (40° F.) to about 16° C. (60° F.). After the water cooling step,another air cooling step may optionally be used. This is usually a shortstep during which the equipment dries with heat removed during theevaporation of the water.

The heating and cooling cycle times will depend on the equipment usedand the article molded. Specific factors include the part thickness andmold material. Typical conditions for a 1/8 inch thick part in a steelmold are to heat the part in an oven with air at about 343° C. (650° F.)for about 19 minutes. The part is cooled in ambient temperature forcedair for about 5 minutes and then in a tap water spray at about 10° C.(50° F.) for about 5 minutes. Optionally, the part is cooled in ambienttemperature forced air for an additional 2 minutes.

During the heating and cooling steps, the mold containing the moldedarticle is continually rotated along two perpendicular axes. The rate ofrotation of the mold about each axis is limited by machine capabilityand the shape of the article being molded. A typical range of operationwhich can be used with the present invention is to have a ratio ofrotation of the major axis to the minor axis of from 1:2 to 1:10, and2:1 to 10:1.

Preferably, the rotational molding is conducted under an inertatmosphere within the mold. However, this is not necessary, although itis preferred when using the preferred composition containingpolyepsiloncaprolactam having an excess of chain ends terminated by adicarboxylic acid group. When it is desired to use an inert atmosphere,the mold cavity can be purged with nitrogen. Most rotational moldingmachines have arms which are drilled for gas injection, so that all thatis necessary is to connect a nitrogen gas cylinder to the arm, using therotary valves in the connection. Alternatively, dry ice can be added tothe mold cavity at the time the resin is charged to the mold. The dryice will sublime during the heating cycle and provide an inertatmosphere.

The mold surface can be sprayed or coated with a mold release coating. Apreferred mold release agent is a baked-on silicon based mold releasecoating, such as Freekote®. This coating is baked for 15-20 minutes atabout 282° C. (450° F.) to 343° C. (550° F.). The mold is cooled andsanded on the inside. The sanded surface allows the composition tomaintain contact during molding and not release to readily and warpwhile being cooled. Yet upon cooling, the molded article easilyreleases. A vent can be used to avoid pressure buildup on the inside ofthe mold during the heating step.

In summary, the mold is charged with a preweighed amount of thecomposition. The mold is closed and clamped securely to be sure thatthere is an adequate mating of the mold at the parting line. Preferably,an inert gas such as nitrogen is fed into the mold. The mold biaxiallyrotates as the molding begins and continues through the heating andcooling steps.

Several examples are set forth below to illustrate the nature of theinvention and the manner of carrying it out. However, the inventionshould not be considered as being limited to the details thereof. Allparts are percent by weight unless otherwise indicated. The physicaltest results were obtained using the following procedures: drop weightimpact--ASTM D-3029 with a 3 inch specimen support; tensile strength atyield, and elongation at break--ASTM D-638; flexural strength andflexural modulus--ASTM D-790; notched Izod impact strength--ASTM D-256;and heat deflection temperature--ASTM D-648. For comparison purposes, inconsidering the drop weight impact values, the drop weight impact of arotationally molded sample of polyepsiloncaprolactam containing nocopper or potassium cations is about 20 ft. lbs. to about 40 ft. lbs.

EXAMPLES 1-6

The polyamide used in Examples 1-6, which follow, ispolyepsiloncaprolactam having more chain ends terminated with azelaicacid than amine groups. The polyepsiloncaprolactam has about 45 milliequivalent (m.eg.) of carboxyl and about 20 m.eg. amine terminated endgroups per kilogram (kg) of polyepsilioncaprolactam. Thepolyepsiloncaprolactam has an estimated formic acid relative viscosityof from about 60 to about 80. The polyepsiloncaprolactam was washed inwater to remove unreacted caprolactam monomer. About 1.5 percentcaprolactam remains. The polyepsiloncaprolactam was impregnated in thewater bath with cupric acetate monohydrate and potassium iodide so thatthere is 0.0025 weight percent of cupric ion in the form of cupricacetate and 0.07 weight percent potassium iodide.

EXAMPLE 1

A cuprous iodide concentrate or masterbatch useful in preparing thepolyespsiloncaprolactam composition of the present invention wasprepared. The composition was made using 98.1 percent by weightpolyepsiloncaprolactam having an excess of chain ends terminated withcarboxyl groups rather than amine groups. 1.8 percent of cuprous iodideand 0.1 percent of Nujol™, mineral oil were used. Thepolyepsiloncaprolactam was charged to a tumbler and the Nujol added.After 2 minutes, the cuprous iodide was added and the material blendedfor 10 additional minutes. The material had an estimated formic acidviscosity of 60 to 80.

EXAMPLE 2

A compostion of the present invention was made using the masterbatch ofExample 1. The composition contained 99 percent polyepsiloncaprolactamhaving an excess of chain ends terminated with carboxyl groups ratherthan amine groups. There was 1 percent by weight of cuprous iodidemasterbatch of Example 1 or 0.018 percent by weight cuprous iodide. Theingredients were physically blended and then extruded in a 41/2 inchbarrel diameter single screw Sterling extruder using a 20 hole by 1/8inch die, and a 200 mesh screen pack. The temperature in the extruderdid not exceed 285° C. Nitrogen was used to blanket the throat of theextruder. The formic acid relative viscosity was estimated to be 60 to80. There were no more than 2 percent extractables and pellets were made1/16 inch in diameter by 1/8 inch long. The pellets were dried to lessthan 0.10 percent moisture. Test specimens were satisfactorilyrotationally molded from this material.

EXAMPLE 3

A composition was made of 99.99 percent by weight polyepsiloncaprolactamresin having an excess of chain ends terminated with carboxyl groupsover chain ends terminated by amine groups, and combined with 0.011weight percent cuprous iodide. The ingredients were blended for 15minutes in a tumbler. They were extruded in a 31/2 diameter single screwHartig extruder using a 20 hole by 1/8 inch die, 50 mesh screen pack.Zone 1 was at 305° C.; zone 2 was at 280° C.; zone 3 was at 260° C.; andthe die was at 250° C. The screw rotated at 60 rpm. The material had aformic acid relative viscosity of 65 to 73 and 0.15 percent moisture.There were no more than 2 percent extractables and the pellet size was1/8 inch long by 1/16 inch in diameter. Test specimens weresatisfactorily rotationally molded from this material.

EXAMPLE 4

The following properties are typical for a dry as molded specimen cutfrom rotational molded parts using the composition of Example 2. Theparts were molded for 19 minutes at 343° C. (650° F.) to a thickness of1/8 inch. The cooling cycle was for 5 minutes in forced ambient air, 5minutes in cold tap water and finally, 2 minutes in forced ambient air.They were molded with the inside of the mold continuously purged withnitrogen.

                  TABLE I                                                         ______________________________________                                        Properties                Values                                              ______________________________________                                        Melting Point (°F.)    420                                             Specific Gravity              1.13                                            Water Absorption, 24 hrs., %  1.3                                             Coefficient of Linear Thermal 4.1 × 10.sup.-5                           Expansion, in./in./°F.                                                 Rockwell Hardness (R scale)   116                                             Tensile Strength at Yield, psi                                                                  21° C. (70° F.)                                                             10500                                           Elongation at Break, %                                                                          21° C.                                                                             20                                              Flexural Strength, psi                                                                          21° C.                                                                             14500                                           Flexural Modulus, psi                                                                           21° C.                                                                             380,000                                         Izod Impact Str., notched,                                                                      21° C.                                                                             0.9                                             ft. lbs./in. notch                                                            Drop Weight Impact ft. lbs.                                                                     21° C.                                                                             70-120                                          Heat Deflection Temp. °F. @                                                              66 psi      350                                                               264 psi     145                                             ______________________________________                                    

EXAMPLE 5

This example illustrates the use of the polyepsiloncaprolactam pluscuprous iodide composition of Example 2 used in the rotational moldingof a 16-gallon tank. In this case, the major axis was rotated at 6 rpmand the ratio of rotation of the major axis to the minor axis was 4:1.The composition contained 99.98 percent polyepsiloncaprolactam and 0.018percent cuprous iodide. A nitrogen atmosphere was used. The 16-gallontank produced had a satisfactory appearance and satisfactory physicalproperties including impact properties, as summarized in Table II below.

EXAMPLE 6

A 160-gallon tank having the dimensions and properties summarized inTable II below was rotationally molded using the composition of Example2. The process conditions used are noted and the rotation of the majoraxis was 6 rpm with the ratio of the rotation of the major to minor axesbeing 4:1. In this process, a nitrogen atmosphere was used. The tank hadsatisfactory physical properties including impact properties, andsatisfactory appearance.

                  TABLE II                                                        ______________________________________                                                       Ex. 5     Ex. 6                                                ______________________________________                                        Tank Size Gals.  16          160                                              Part. Thickness, in.                                                                           0.150       0.250                                            Part. Dimensions, in                                                                           25 × 25 × 10                                                                  60 × 60 × 18                         Part. Weight, lbs.                                                                             14          150                                              MOLDING CONDITIONS                                                            Oven Temperature, °F.                                                                   650         650                                              Heating Cycle, min.                                                                            17          28                                               Cooling Cycle, min.                                                           Air              5           5                                                Water            3           8                                                Air              2           2                                                Rotation - Major, rpm                                                                          6           6                                                Ratio            4:1         4:1                                              Inert Atmosphere Yes         Yes                                              ______________________________________                                    

In Examples 7-25 which follow, the compositions were blended andpelletized using a laboratory size 21/2 inch diameter NRM single screwextruder. Cubes 16 inches on each side having a 1/8 inch wall thicknesswere rotationally molded in the laboratory to judge appearance and testphysical properties. Generally, impact testing of moldings from thislaboratory extruder has been found to be somewhat erratic compared tothe results when using manufacturing size equipment of the type used inExamples 1-6. The variability is believed to be caused by the higherdegree of mixing obtained when using manufacturing versus laboratorysize extruders.

EXAMPLES 7-8

Examples 7 and 8 summarized in Table III below were conducted withpolyepsiloncaprolactam of the type used in Examples 1-6, containing0.011 percent cuprous iodide. This brought the total amount of copper toabout 60 parts per million, and there were 700 parts per millionpotassium iodide. In Example 8, 8 percent by weight of caprolactammonomer was added as a plasticizer. The samples were rotationally moldedin a CO₂ atmosphere at the summarized conditions. The samples were 1/8inch thick and had satisfactory appearance. Times are in minutes.

                                      TABLE III                                   __________________________________________________________________________                         AIR WATER                                                                              DROP WT.                                        ADDED   ADDED                                                                              OVEN                                                                              OVEN                                                                              COOL                                                                              COOL IMPACT                                          CAPRO.  Cu.sub.2 I.sub.2                                                                   T °C.                                                                      TIME                                                                              TIME                                                                              TIME FT. LBS.                                        __________________________________________________________________________    Ex. 7                                                                            0    0.011%                                                                             343 19  5   5    92                                              Ex. 8                                                                            8    0.011%                                                                             343 19  5   5    92                                              __________________________________________________________________________

These results indicate that the use of cuprous iodide with the potassiumiodide results in a composition which has satisfactory appearance andexcellent impact properties both with and without added caprolactamplasticizer. In the past, rotationally molded polyamide preferablycontained a plasticizer, such as the monomer, to maintain higher impactproperties. This is not necessary with the composition of the presentinvention. However, a plasticizer can be used if desirable to improvethe flexibility of the molded article.

EXAMPLES 9-13

Samples were prepared from polyepsiloncaprolactam of the type used inExamples 1-6 and containing 0.018 percent cuprous iodide melt blended,at a variety of extrusion conditions. The extrusion melt temperatureswere varied and different size screen packs were used. A screen pack isa screen located behind the die. The screen is supported by a breakerplate which has a plurality of larger holes to break up the materialcoming from the screw. The screen increases the back pressure in theextruder and results in more thorough mixing. The compositions wereextruded to form cylindrical pellets 1/16 inch in diameter by 1/8 inchlong. The pellets were rotationally molded to form 16 inch cubes 1/8inch thick. Samples from these cubes were tested for drop weight impact.The cubes were rotationally molded in a CO₂ atmosphere for 19 minutes at343° C. (650° F.). The mold is then cooled in forced ambient air for 5minutes, in a water spray at cold water tap temperature for 5 minutes,and in forced ambient air cooled for 2 minutes. The water used was atthe temperature of the cold water supply to the laboratory, about 10°(50° F.). The mold rotated along two axes simultaneously with a rotationratio of 4 to 1 from one axis to the other. The major axis of rotationwas at about 6 revolutions per minute. The extrusion temperature, screensize, drop weight impact value and failure mode are summarized in TableIV below:

                  TABLE IV                                                        ______________________________________                                        Ex-  Extr. Melt                                                                              Screen Size                                                                             Drop Wt.                                                                              Number Number                                am-  Temp.     (Mesh     Impact  Shatter                                                                              Ductile                               ple  °C.                                                                           (°F.)                                                                         Size)   ft-lbs. Failures                                                                             Failures                            ______________________________________                                         9   232    (450)  200     103     5      3                                   10   260    (500)  200     120     5      5                                   11   302    (575)  200      30     14     0                                   12   260    (500)  200     103     3      6                                   13   260    (500)   50      54     11     0                                   ______________________________________                                    

These results indicate that when melt blending the cuprous iodide intothe polyepsiloncaprolactam, a temperature in the extruder of 302° C.results in decreased impact values. Additionally, Examples 12 and 13indicate that the increased back pressure by use of a 200 mesh screenrather than a 50 mesh screen improved the impact resistance. It is notedthat ductile type failure is preferred to a shatter type failure. Theductile failure is more local puncture, while a shatter results incracking over a larger area. The use of a lower extruder temperature andthe use of a finer screen both result in improved impact resistance.

EXAMPLES 14-18

Table V below summarizes a study to determine the importance ofpotassium iodide to the appearance of a rotationally molded article.Compositions of unwashed polyepsiloncaprolactam were used. The unwashedpolyepsiloncaprolactam contained about 10 percent of a caprolactammonomer and no copper or potassium cations other than those indicated inTable IV. An article having good appearance has a uniform, light naturalwhite color. Articles having bad appearance had dark specks. The sampleswere prepared in the same manner as those in Examples 9-13. The cupricacetate was added as cupric acetate monohydrate. The potassium iodidewas added in crystal form, and also in powder form mixed with onepercent Cab-O-Sil™, a colloidal pyrogenic silica, to improve its flow.The amounts of material used are in grams.

                  TABLE V                                                         ______________________________________                                                   Ex. 14                                                                              Ex. 15  Ex. 16  Ex. 17                                                                              Ex. 18                                 ______________________________________                                        Polyepcapr.  5928    5939    5922  5922  5933                                 (10% capr)                                                                    Cu.sub.2 I.sub.2                                                                           72      60      72    72    60                                   Cu (C.sub.2 H.sub.3 O.sub.2).sub.2.H.sub.2 O                                               --      .47                 .47                                  KI (crystal) --      --            6     6                                    KI(1% + Cab-O-Sil)                                                                         --      --      6                                                Appearance   specks  specks  good  good  good                                 ______________________________________                                    

EXAMPLES 19-25

Table VI below summarizes a study of the appearance of variousformulations. In Examples 19-25 unwashed polyepsiloncaprolactam was usedwhich contained no copper or potassium compounds other than that shown.The unwashed polyepsiloncaprolactam contained about 10 percent by weightof caprolactam monomer. In Examples 24 and 25 the polyepsiloncaprolactamused was of the type used in Examples 1-6 containing only about 1.5percent by weight of caprolactam and about 0.07 percent by weight ofpotassium iodide and about 0.0025 percent by weight of cupric acetatemonohydrate (Cu(C₂ H₃ O₂)₂.H₂ O). The samples in Examples 19-25 wereprepared in the same manner as those in Examples 9-13. The amounts ofeach material are in grams.

                                      TABLE VI                                    __________________________________________________________________________              Ex. 19                                                                            Ex. 20                                                                            Ex. 21                                                                            Ex. 22                                                                            Ex. 23                                                                            Ex. 24                                                                            Ex. 25                                      __________________________________________________________________________    Polyep. capr.sup.1                                                                      10000                                                                             9880                                                                              9889                                                                              9930                                                                              9997                                                Polyep. capr.sup.2            9900                                                                              9200                                        Cu.sub.2 I.sub.2                                                                            120 100         120 100                                         CuCl.sub.2            70                                                      Cu(C.sub.2 H.sub.3 O.sub.2).sub.2.H.sub.2 O                                                     .79     2.67                                                Caprolactam                       700                                         KI (1% Cab-O-Sil) 10                                                          Appearance                                                                              Good                                                                              Specks                                                                            Good                                                                              Good                                                                              Purple                                                                            Good                                                                              Good                                        __________________________________________________________________________     .sup.1 Polyepsiloncaprolactam with 10 percent caprolactam monomer.            .sup.2 Polyepsiloncaprolactam with 1.5 percent caprolactam monomer, 0.002     percent Cu(C.sub.2 H.sub.3 O.sub.2).sub.2.H.sub.2 O, and 0.07 percent         potassium iodide.                                                        

Examples 14 through 25 indicate that unwashed polyepsiloncaprolactam,which contains about 10 percent caprolactam monomer, requires potassiumiodide for a good appearance when using cuprous iodide. The use of thecupric acetate alone resulted in a purple colored molding.

While exemplary embodiments of the invention have been described, thetrue scope of the invention is to be determined from the followingclaims:

What is claimed is:
 1. A method of rotationally molding a polyamidearticle in a rotationally molding mold comprising rotationally molding apolyamide composition in the presence of oxygen wherein the compositioncomprises a polyamide having from 60 percent to 90 percent of thepolyamide chain ends terminated with acid groups, and from about 0.001percent to about 0.5 percent, by weight of the polyamide, of a coppercompound.
 2. The method as claimed in claim 1 further comprising meltblending the polyamide composition at a temperature higher than the melttemperature of the polyamide and less than about 282° C. (540° F.). 3.The method as recited in claim 2 wherein the step of rotationallymolding further comprises:feeding the polyamide composition to thecavity of the rotational mold, wherein the composition comprises apolyamide and from about 0.01 percent to about 0.5 percent, by weight ofthe polyamide, of a copper compound; rotating the mold simultaneouslyaround two perpendicular axes; heating the composition within the mold;cooling the mold; and removing the molded article.
 4. The method asrecited in claim 2 the preparation of the polyamide composition by thefurther steps of:physically mixing the polyamide composition and thecopper compound prior to melt blending.
 5. The method as recited inclaim 1 wherein the copper compound is cuprous iodide.
 6. The method asrecited in claim 1 wherein the composition further comprises potassiumiodide.
 7. The method as recited in claim 1 wherein the polyamide hasfrom 60 to 80 percent of the chain ends terminated with acid groups. 8.The method as recited in claim 1 wherein the polyamide ispolyepsiloncaprolactam.
 9. The method as recited in claim 8 wherein thepolyamide has from 60 to 80 percent of the chain ends terminated withacid groups.
 10. The method as recited in claim 8 wherein thecomposition further comprises about 8 percent to about 12 percentcaprolactam monomer.
 11. The method as recited in claim 2 wherein thepolyamide is melt blended at a temperature higher than the melttemperature of the polyamide and less than about 500° F.
 12. The methodas recited in claim 1 wherein the composition is rotationally molded inthe presence of air.